Seeking expert testimony is a sign of strength. A personal relationship with a caring professional is proven clinically effective.
I believe that I can help with your situation.
This has surely been very devestating news for you and must weigh very heavily on you.
Your fiance has more dimesnions than he ever told you about. Perhaps some of his preferences are new, and perhaps tied to the fact that you have stopped having sex with him, but perhaps because he has a strong drive and lots of sexual fantasies and needs something like this in his life.
He may have been this way after you were married, and if you hadn't discovered this other side then it might have continued into your marriage.
Of course it is udermining your desire to marry him, because he has betrayed you emotionally if not physically, and he is willing to take a risk of bring sexually transmitted diseases into your life.
You cannot continue to ignore this. It seems that you will have to contront this and probably should be prepared, before you say anything, to line up a good family therapist, or preferably, a sex therapist who is a licensed mental health professional with additional training and certification.
I know you don't want to deal with a therapist but the two of you must face one.
You should postpone your wedding until you sort this out.
Best place to find the most highly qualified sex therapists are at AASECT.AASECT - Member Directory
You are not prepared to be married now and must sort this out first.
Set up an appointment. Confront your fiance and be prepared to go together, or if you can arrange it, confront him at the therapists.
You cannot marrry successfully until you work this out.
I wish you courage and shall keep you in my prayers.
Elliott, MAE, LPCC, NCC, CCMHC